Emergency-call-answering centers, such as E911 call-answering centers, are designed to handle a given traffic load based on the average busiest-hour call volume. Trunk quantities and call-answering staff requirements are determined from this data. If the trunk quantities are increased in the actual installation to provide redundant trunking, or if the required staff is not available to handle the offered load, the center is exposed to a possible overload situation. In addition to this problem, emergency-call-answering centers are typically faced with requirements to answer ninety percent of all emergency calls within approximately two rings while being faced with unpredictable traffic peaks.
The public network provides some overload protection, but only in the event that all trunks leading to the call-answering center are busy. In that case, the network routes additional incoming emergency calls to a pre-designated alternate call-answering center. An administrator of an emergency call-answering center can also manually cause all further calls to be switched by the network to an alternate call-answering center. But this facility is only useful if the primary center is being placed out-of-service. Thus, call-answering centers are faced with the difficult task of handling traffic surges without any facility for dynamic-overload protection.
One known system is understood to have attempted to overcome this problem by means of a call-forward busy/don't answer feature, whereby it reroutes calls, that have been queued up and waiting to be answered by a primary call-answering center for longer than a predetermined time (10 seconds), to an alternate call-answering center. However, this approach has a significant disadvantage, which may have tragic consequences in the case of an emergency call: a call must undergo the 10-second delay of first waiting in the queue of the primary call-answering center before being switched to the alternate call-answering center to be answered. This disadvantage becomes especially acute if the alternate call-answering center is also busy, and the call must be switched to yet-another call-answering center, after now having waited in two queues; the delays are cumulative.